Ch. 3 ATOMIC STRUCTURE
Sizing up the Atom • Elements are able to be subdivided into smaller and smaller particles – these are the atoms, and they still have properties of that element • If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long • Despite their small size, individual atoms are observable with instruments such asscanning tunneling (electron) microscopes
AnSTM image of nickel atoms placed on a copper surface. Source: IBM Research
Image of a ring of cobalt atoms placed on a copper surface. Source: IBM Research
Atom - smallest particle making up elements One teaspoon of water has 3 times as many atoms as the Atlantic Ocean has teaspoons of water!
Think about the technological advances of the past 100 years! They have been nothing short of miraculous! Radios Calculators Televisions Computers Automobiles Cell phones Jet airplanes Ipods Plastic Velcro Refrigerators Internet (thanks, Al Gore) Penicillin CD’s & DVD’s Insulin and, of course - Electric guitars Sliced Bread!
Where did it all begin? The word “atom” comes from the Greek word “atomos” which means indivisible. The idea that all matter is made up of atoms wasfirst proposed by the Greek philosopher Democritus in the 5th century B.C.
Then came the idea of “The 4 Basic Elements” Earth, Air, Fire, & Water After that came Alchemy. The change to “real” Chemistry didn’t occur until the first true element was discovered! (1774) The first element discovered was
The discovery of oxygen is attributed to 3 scientists (working independently) • Karl Scheele (1771) (German) • first to prepare and describe oxygen • Joseph Priestley (1774) (British) • isolated oxygen gas from mercuric oxide. • observed accelerated burning • Antoine Lavoisier (1784) (French) • made accurate measurements and interpreted Priestley’s results
Carl Wilhelm Scheele beat Priestley to the discovery but published afterwards. Too bad! – So sad!
Priestley Medal Priestley gets the main credit for discovering oxygen! Source: Roald Hoffman, Cornell University
2HgO(s) → 2Hg(l) + O2(g) • Priestley produced a gas (oxygen) by using sunlight to heat mercuric oxide kept in a closed container. The oxygen forced some of the mercury out of the jar as it was produced, increasing the volume about five times.
Priestley: Scientific Contributions DISCOVERY OF 8 GASES • Oxygen • Nitrogen • Carbon Dioxide • Carbon Monoxide • Sulfur Dioxide • Nitrous Oxide • Nitric Oxide • Hydrogen Chloride
Priestley: Additional Scientific Contributions • Discovered the interconnection between photosynthesis and respiration • Discovered carbonated water • Discovered that India rubber removed graphite pencil marks - the first rubber eraser Now we can make mistakes!!
Lavoisier: the Founder of Modern Chemistry • Lavoisier continued the investigations of Priestly • Quantitative experiments led to: • Law of Conservation of Matter. • He systematized the language of chemistry, its nomenclature and rhetoric. • He was beheaded during the Reign of Terror for his role as a tax “farmer” prior to the Revolution (Priestley escaped to America!) Antoine-Laurent Lavoisier
Lavoisier heated a measured amount of mercury to form the red mercuric oxide. He measured the amount of oxygen removed from the jar and the amount of red oxide formed. When the reaction was reversed, he found the original amounts of mercury and oxygen. 2Hg(l) + O2(g) → 2HgO(s)
Properties of Oxygen P PPPPCC • Colorless • Odorless • Tasteless • Gas at room temperature • Slightly soluble in water • Inflammable (does NOT burn) • Only part of air that supports combustion Physical Property or Chemical Property?
These properties of oxygen were later used to determine the properties of other substances. By the late 18th century, scientists finally came to the conclusion that Oxygen was truly an element (can’t be broken down into simpler forms without losing its properties) Scientists began to search for & test other new elements.
Sometimes, when they tried to react substances together, nothing happened! Substances that DO NOT react are Inert They found that most materials will react to form new substances. These elements are said to be chemically active (reactive) Oxygen is very reactive, so is hydrogen which we will look at next! Oxygen hydrogen inert Increasing chemical reactivity
Discovery of • Henry Cavendish (1766) • Reacted various metals with acids producing a salt and hydrogen gas • Acid + metal → hydrogen gas + salt • Zinc + sulfuric acid → Hydrogen + zinc sulfate • Zn(s) + H2SO4(aq) → H2(g) + ZnSO4(aq) • While testing the properties of Hydrogen he • found that water is a compound (1731 – 1810) Word Equation Chemical equation
Hydrogen + Oxygen Water 2H2 + O2 2H2O
Antoine Lavoisier • Named Priestly’s newly discovered gas - “oxygen” - meaning “acid former” • Named Cavendish’s new gas “hydrogen” -meaning “water former”
Dalton’s Atomic Theory John Dalton (1766-1844) While his theory was not completely correct, it revolutionized how chemists looked at matter and brought about chemistry as we know it today (instead of alchemy) So, it’s an important landmark in the history of science.
Dalton’s Modern Atomic Theory(experiment based!) All elements are composed of tiny indivisible particles called atoms Atoms of the same element are identical. Atoms of any one element are different from those of any other element. Atoms of different elements combine in simple whole-number ratios to form chemical compounds In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element.
Law of Definite Proportions • Each compound has a specific ratio of elements by mass. • Ex: Water is always 8 grams of oxygen for each gram of hydrogen.
Discovery of the Electron Began with the invention of the Crooke’s Tube (cathode ray tube) c. 1875
Voltage source Cathode RayTube • Electric current sent through gases sealed in tube at low pressure • Anode- positive electrode • Cathode- negative electrode + - gas Metal Disks - electrodes
Modern Cathode Ray Tubes Television Computer Monitor • Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.
In 1897, J.J. Thomson used a cathode ray tube to study gases.
Voltage source Thomson’s Experiment - + • Passing an electric current makes a beam appear to move from the negative to the positive end – so the ‘beam’ was called a “Cathode Ray”
Voltage source Thomson’s Experiment - • Thomson found that cathode rays were deflected from a negatively-charged plate.
Voltage source Thomson’s Experiment + • and that cathode rays were attracted to plates with a positive charge • Does light bend like this?
Light doesn’t ‘bend’ so the cathode ray must be made of particles rather than Light! Since they are attracted to a positive plate & repelled by a negative one the particles aren’t neutral – What charge must they have? That’s right! NEGATIVE!! Thomson called thesenegative particles – ELECTRONS
Mass of the Electron Mass of the electron is 9.11 x 10-28 g The oil drop apparatus 1916 – Robert Millikan determined the mass of the electron: 1/1840 the mass of a hydrogen atom; and, has one unit of negative charge
Conclusions from the Study of the Electron: Cathode rays have identical properties regardless of the element used to produce them. Therefore, all elements must contain identically charged electrons. Atoms are neutral, so there must be a positive substancein the atom to balance the negative charge of the electrons Electrons have so little mass that atoms must contain other particles that account for most of their mass
Thomson’s Atomic Model J. J. Thomson Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.
In 1903, An important discovery leading to further understandings of atomic structure happened by accident. Henri Becquerel discovered radioactivity Radioactivity is the spontaneous emission of energy from an object 1903: Shared a Nobel Prize with Pierre and Marie Curiefor discovering radioactivity.
Ernest Rutherford (1871-1937) The Nobel Prize in Chemistry 1908 Studied under J. J. Thomson
3 Types of Radiation discovered by Ernest Rutherford • Alpha (ά) – a positively charged helium nucleus42 He+2 • Beta (β) – fast-moving electrons -e • Gamma (γ) – like high-energy • x-rays
Ernest Rutherford’sGold Foil Experiment - 1911 • Shot alpha particles at a thin sheet of gold foil • Particles that hit on a detecting screen (film) were recorded
Flourescent Screen Lead block Polonium Gold Foil
He Expected: • The alpha particles to pass through the foil without changing direction very much. • Because… • The positive charges were spread out evenly (according to Thomson’s atomic theory). Alone they were not enough to stop the alpha particles.
Rutherford’s Observations “Like howitzer shells bouncing off of tissue paper!” • Most of the particles went straight through the foil (what he expected) • A few particles were slightly deflected • Still fewer actually bounced back towards the source! • Astonishing!!! • Rutherford said it was like firing a Howitzer shell at a piece of tissue paper & having it bounce back & hit you!
+ Rutherford’s Conclusions • Since most of the particles went through the foil - atoms are mostly empty space. • Because a few +particles were deflected they must have come close to a positively charged core. • Since a very few particles were deflected straight back, the positively-charged core must be very dense. • This small dense positive area is the nucleus.